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338                               CHEMICAL ENGINEERING
beaters come above the surface of the water. Although these machines run more slowly than impeller types with vertical shaft the wear and tear on the beaters is more severe than on the impeller type. In this type the impeller not only serves for beating in air but it has the further function of pulling the pulp, except the portion which rises as froth, from the spitzkasten or compartments facing the impeller compartments provided for froth rising, to the impeller compartment. In the multi-compartment machines employing the Minerals Separation process the pulp passes into the first impeller compartment and from this it flows into the first spitzkasten where the first froth rises. The general level of the impeller compartments is the same throughout as is also that of the spitzkasten but that of the latter is lower than the impeller compartments. The material which does not float in the first spitzkasten is drawn up into the second impeller compartment by the pull of its impeller. It then flows into the second compartment where second "floats" are obtained the balance being pulled to the third impeller compartment, etc.
This allows of varying the treatment in various compartments, routing of "floats" of the different grades from the different compartments to different points for separate disposal, advantages which are not possessed by the horizontal beater machines unless they be used in series with consequent loss of head room and multiplication of apparatus. The horizontal machine has the advantage that it takes in the air better and it is no difficult matter to secure any desired regulation. The vertical impeller type with high velocity of rotation or Minerals Separation type makes a much stronger elastic froth than any other type of machine and on high ratio of concentration problems this is a very desirable feature, particularly if the minerals to be separated are of high specific gravity. A strong froth is not so desirable a feature where there is present in the ore a large proportion of sulphides of relatively low specific gravity as the lack of air in many cases will make for low capacity. The tendency of the Minerals Separation Co.'s engineers has been to make the arrangements of the impeller compartment such that the pulp in it is as shallow as possible. This increases the entrapping of the air but affords no means of regulating it.
The general arrangements for a 12-cell Minerals Separation type of machine are shown in Fig. 9. A peripheral speed of 1,200 ft. per minute or thereabouts is best for the impellers. Four of these machines used as roughers handled up to 500 tons per day of 24 hr. This is a machine of small-size cell and was used on a high ratio of concentration problem. On a low ratio problem this spread of surface would probably not take care of more than 300 tons per day and fewer units and larger cells would be preferable. The froth material from the roughers went to two eight-cell machines of identical design. The waste from the finishers went to one 12-cell machine and the waste from this was returned to the 4 roughers for retreatment. The flotation concentrate passed to two pilot tables and the crystalline concentrates made were routed to two settling tanks from which they alternately drawn daily and shipped. The colloidal concentrate was pumped into a 30-ft. Dorr thickener and finished on a Portland filter.
Other Working Data on Minerals Separation Machines.—If 12-in. impellers are employed the power required to drive one impeller is about 1.75 hp. Larger size impellers may be reckoned as being proportional to the diameter, using the horsepower figure given as a base. The consumption in power with other types of machine is less than this.
Oils.—These are of two kinds the "collectors" which have high affinities for sulphides and the "frothers." Hard-wood creosote is the standard collective oil but owing to its expense other wood-, coal-, and oil-distillate products are substituted for it either entirely or in part, coal tar being a favorite. On silver minerals